It is common practice in the completion of subterranean wells, such as oil and gas production wells, to include subsurface well tools and devices. These devices can be remotely controlled or operated via a small diameter conduit or control line. One common example of a downhole tool of this type is a subsurface safety valve. Safety valves are used to shut off the flow of fluid in the well production tubing. Subsurface safety valves can be controlled or operated in response to fluid pressure conducted through a control line to the valve from the well surface. A controller is located at the well surface and is typically designed to respond to emergency conditions, such as fire, broken flow lines, oil spills, etc., to actuate the safety valve to shut off flow from the well.
In a typical environment, a tubing mounted safety valve is made up in the tubing string and installed in the well tubing with a hydraulic control line extending to the surface along the outside of the tubing string. In cased wells the control lines are located in the space formed between the tubing and casing. Examples of tubing retrieval safety valves are described in U.S. Pat. No. 4,945,993 or can be purchased from the Halliburton Company, 2601 Beltline Road, Carrollton, Tex. 75006 as the series 10 W flapper-type valves. The patents listed herein are incorporated by reference for all purposes in this application.
In environments where a well is anticipated to have a long life, provision is made for inserting a second subsurface safety valve in the well at a later time. To accomplish this, typically, a safety valve landing nipple is placed in the well tubing at the time of installation of the tubing retrieval safety valve. A landing nipple is a device that is designed to receive a downhole tool such as a surface controlled retrievable safety valve. Typically, landing nipples for safety valves have a structure mating with a lock mandrel for mounting the safety valve in place have and a port or passageway extending through the wall of the nipple connected to a hydraulic control line extending to the surface. The control line connected to the landing nipple can be used at a later date to control a safety valve or other device mounted in the nipple. If the primary well tubing safety valve becomes unreliable a second safety valve, can be installed in the landing nipple. Typically, safety valves of this type are installed using wireline, pump down, or other means as are well known in the industry. Retrievable landing nipples, lock mandrels, and safety valves therefor are shown in U.S. Pat. No. 5,323,859, and devices of this type can be purchased from Halliburton Company as XXO, FRQ, and KRQ landing nipples.
It has been found that in wells where there is a long time interval between initial installation of the landing nipple and the ultimate installation of the safety valve that the hydraulic line connected thereto can become irreversibly clogged with well fluids or well treatment fluids that have moved into the hydraulic control line through the port in the landing nipple. In those situations where the hydraulic line is clogged, an expensive process to install a new safety valve must be performed, and the well must be shut down during the process. Similar contamination and plugging problems with open control lines are present in other downhole devices such as landing nipples for sliding doors and the like.